Cardiac Output

If you only learn one thing from the cardiac content, make sure it's this:

Cardiac Output = Heart Rate x Stroke Volume

It's kind of a big deal.

But what does it mean, really? 

Well, cardiac output as defined by the Iggy textbook is the measurement of blood flow from the heart into the systemic arterial circulation. You can also think of it as the amount of blood pumped from the left ventricle each minute. 

It helps to think of it like this: Cardiac Output is the full measurement of what equals a healthy amount of bloodflow. Bad cardiac output? Unhealthy amount of bloodflow. Your body's tissues and organs will suffer.
Kind of simple, but it get's the point across.

  If it helps, watch the above image for a full minute. Every time that left ventricle contracts, imagine blood is pumping to the heart and the rest of the body. Important principle, right?

CO = HR x SV

(I'm a firm believer that repetition makes things stick better, so expect to see that a lot!) 

Now, cardiac output is affected by heart rate and stroke volume. So let's break it down.

Heart Rate

If you want a more "official" definition, heart rate is the number of times the ventricles contract each minute. Normally, it's around 60 to 100. You can find that in the Iggy text on page 707.

So, heart rate is really as simple as it sounds - count the number of heart beats in a minute. Depending on the speed of your internet, the gif above should have about 60 beats per minute (bpm). There's your heart rate.

Naturally, increases in heart rate will increase myocardial (heart) tissue oxygen demand. Think of it like this: If you suddenly refused to drive your car under 100mph, you can expect your car to need more gas more often, right? Similar principle.

So, what if you were unable to get that extra gas? Let's say you drove so fast and hard you're now out in the middle of a desert. No gas station for miles and you're running on empty. You're not going to be able to continue to drive 100 mph anymore. 

Now, imagine that car is your heart - the gas is oxygen. For whatever reason, you can't take in enough oxygen to meet the needs of your heart. Your heart will at first try to compensate, but after a while we expect to see a decrease in cardiac output. And what's cardiac output again?

The full measurement of what equals a healthy bloodflow!

CO = HR x SV

Stroke Volume

Stroke volume is the amount of blood ejected by the left ventricle during each contraction (Iggy page 707). It's the difference between end-diastolic volume (the amount of blood during the relaxation and filling of the left ventricle before it contracts) and end-systolic volume (the amount of blood left after the contraction and emptying of the left ventricle).

Stroke Volume is affected by many different variable, (such as preload, afterload, and contractility) but we'll focus on Preload and Afterload for now.






Preload is the degree of myocardial fiber stretch at the end of diastole and just before contraction.


Meaning...


It measures the elasticity of the cardiac cells when the muscle fibers in the left ventricle are stretched by the volume of blood they're holding. Anatomy time!


Cardiac muscle exhibits a length-tension relationship. What that means is that resting muscle fibers in cardiac muscle are kept shorter than the defined 'optimal length' so that they are able to develop maximum tension. Therefore, stretching cardiac cells can produce dramatic increases in contractile force.


Too wordy? Yeah, it is. Here's an easier way to look at it.
Pretend the cardiac muscle fibers are rubber bands. You take a rubber band and stretch it between two fingers. Suddenly, you let it go. 

If you stretched it about this distance, it's not going to go very far, right? 

Instead, take that rubber band and stretch it with the thumb of one hand and the index finger of the other.

If you let that one go, its going to have some distance, right? Same principle with Preload. If each muscle fiber gets stretched to capacity by the filling of blood, they're going to be able to shoot the blood out to the rest of the body with enough force to perfuse all the tissues. The more the heart fills during diastole (within limits, of course) the better and more forceful the contractions will be.

Afterload is the pressure or resistance that the ventricles must overcome to eject blood through the aortic semilunar valve and into the peripheral blood vessels.

The amount of resistance is directly related to arterial blood pressure and the diameter of the blood vessels. (Iggy page 708)

So what does that mean? 

Well, your valves have to shut pretty tight. Otherwise you'll see leakage back into the atria, or blood will leak through into the aorta before the ventricles contract properly. In both instances, blood isn't going where it needs to be. What that means for the ventricles is they have to overcome the pressure that's holding the valves so tightly closed so that blood can be ejected out and go the to rest of the body - where it needs to be.

If it helps, you can picture afterload as a garden hose. There has to be enough pressure within the hose to force the water through and out. So, let's say that arterial blood pressure is represented by the hose's spigot.

The spigot puts pressure on the water, forcing it out into either a strong stream (high pressure) or tiny dribble (low pressure).

Now, let's say that the hose itself represents the blood vessels. If you've got a good clean hose, it's going to allow good water flow. If you have an old hose with mineral build up or other trash inside, that water flow is going to be restricted. Pressure may be high, but the way is blocked. Tiny blood vessels equal poor blood flow.

 So if your ventricles are having to fight against a high amount of resistance from either high arterial pressure and/or decreased blood vessel diameter, how good is your stroke volume going to be? How adequate is your cardiac output going to be?

CO = HR x SV